Detecting leakage from fluid storage tanks is a very important problem. For example, leaks from large liquid fuel storage tanks can be very damaging, and detecting such leaks is important in order to allow repair or preventive measures. Detecting leaks when the leak itself is very small or the leakage rate is slow is very difficult using presently available techniques.
The use of pressure transducers to measure the level of a liquid in a tank by measuring the hydrostatic head pressure of the tank is known. However, detecting a small leak by observing changes in the indicated tank level is very difficult using such pressure measurement techniques. This difficulty is caused by the fact that only very small changes in tank level will be observed, and thus the change in the measured hydrostatic head pressure will be very small due to leakage. Also, temperature changes and level changes due to evaporation may be falsely interpreted as leakage indications. The leakage of fluid from tanks may be very undesirable for a number of reasons, including the wastage of costly tank contents and pollution produced by leakage that creates environmental contamination.
Relevant prior art cited in patent applications referenced above, are Jacobs, U.S. Pat. No. 3,538,746 which shows a similar system but with a submerged differential pressure gauge and electrical conductor 36 subject to hazard if used with combustible fluid. Equal head pressure is not maintained. Also, the bottom of pressure chamber 32 is substantially above the bottom of tank 12 creating temperature errors.
Maresca, Jr., et al., U.S. Pat. No. 4,646,560 maintains equal head pressures but again the DPU is submerged.
Guignard, U.S. Pat. No. 3,460,386, Tavis, U.S. Pat. No. 4,627,281, Morooka, U.S. Pat. No. 4,389,888, Knapff, U.S. Pat. No. 3,537,298, Plegat, U.S. Pat. No. 3,921,436, Alm, U.S. Pat. No. 3,939,383, and Torre, U.S. Pat. No. 3,527,909 were also cited but are considered less pertinent.